Measuring fluctuations and dissipation in an ATP-chemostated system*

Many biological systems act as ATP chemostats, regulating metabolism to maintain the ATP/ADP ratio at constant levels, driving nonequilibrium activities. In chemostats with multiple dissipative processes, it remains unclear how a specific process contributes to the global dissipation of the system. Using actin-intact cytoplasmic extract from Xenopus laevis frog eggs as a model system, we measure how a specific dissipative process - local actomyosin fluctuations – relates to the global dissipation. To estimate local dissipation, we image conformational fluctuations of single-walled carbon nanotubes (SWNTs) embedded in the actomyosin network, and we analyze breaking of detailed balance in the space of normal modes. To estimate global dissipation and the chemical potential gradient, we quantify the global ATP turnover, as well as ATP and ADP concentrations, using fluorimetric assays. To vary the chemical potential, we introduce apyrase, which provides an additional exogenous dissipative pathway by catalyzing the hydrolysis of ATP. We thus reveal how a specific, local dissipative process is a function of the global dissipation of the system.